This invention relates to tie-down devices and, more particularly, to a tie-down device having a strap and a tension indicator for indicating the tension in the strap.
Straps are often used to tie down equipment in moving vehicles such as aircraft and trucks or to hold down stationary equipment which is subject to vibration. Equipment such as tanks, containers, accumulators, and reservoirs are often susceptible to damage by a strap which is wrapped tightly around the equipment. Overtensioning of the strap may also create excessive stress on a bolt connecting two ends of the strap, resulting in failure of the bolt or strap. Insufficient tightening of the bolt may result in loosening of the strap through vibration which would allow the equipment to move from its secured position and possibly cause damage to the equipment or surrounding structure.
Previously, torque wrenches have been used to measure the torque applied to the bolt in an attempt to determine the tension in the strap. However, this method is only an indirect measurement of the tension in the strap and is often inaccurate since torque values do not accurately indicate preload and can change as threads wear or corrode. Furthermore, the torque value obtained is often unreliable and not repeatable since the thread and nut friction account for a large percent of the applied torque. Variations in the coefficient of friction due to machining, surface treatment and surface contamination, such as grease, may further vary the torque reading from the actual tension in the bolt.